The invention relates to switching of an AC motor between power from different sources, such as power from supply mains of a power utility and power from a power converter, such as a variable frequency AC motor drive. More specifically, the invention is a method and apparatus for accomplishing such a transfer at high speed to eliminate the need for a line inductor to control currents.
In many applications, it is desirable to switch an AC motor receiving power directly from supply mains to receive power from a power converter, or vice versa. The phrase xe2x80x9csupply mainsxe2x80x9d, as used herein, refers to any source of AC electric power having a frequency that is determined by the power utility. The term xe2x80x9cpower converterxe2x80x9d as used herein refers to any source of AC electric power having a frequency that is generated by a mechanism other than the power utility, such as an inverter, a three phase AC variable frequency motor drive, a smart motor starter, an uninterruptible power supply or the like.
For example, in gas turbine static starters, a variable frequency motor drive is often used to bring a generator motor up to a self-sustaining speed at which the generator motor can subsequently be switched to supply mains. In other applications, such as variable speed fans and pumps, it is desirable to achieve variable speed with a variable frequency drive and subsequently switch the motor to supply mains for sustained operation at a constant high speed. Also, in the event of failure of the motor drive, it is desirable to switch the motor to the supply mains to continue operation. Similarly, it is often desirable to switch a motor from supply mains back to a motor drive for slowing the motor down or otherwise varying the speed or torque of the motor.
Switching of a motor from a power converter to supply mains is referred to xe2x80x9ctransferxe2x80x9d herein and switching of a motor from supply mains to a power converter is referred to as a xe2x80x9ccapturexe2x80x9d herein. FIG. 1 illustrates a conventional system for transfer and capture. System 10 includes supply mains 12, a source of three phase AC power at frequency f, power converter 14, a variable frequency AC drive for example, drive output contactor 16, utility contactor 18, and motor 20. It can be seen that operation of contactors 18 and 16 can be controlled to selectively couple motor 20 directly to supply mains 12, to power converter 14, or to both.
Conventionally, both power converter 14 and the supply mains 12 are coupled to motor 20 for a brief period of time, i.e. in an overlapping manner, during transfer or capture to provide a smooth transfer or capture procedure. This is known as a xe2x80x9cmake before breakxe2x80x9d transfer or capture. However, this can cause very high currents to flow between power converter 14 and supply mains 12 due to instantaneous voltage differences between the two. If not controlled, the high currents can cause damage to power converter 14 and can cause undesirable operation of the motor 20. Accordingly, an inductor, such as line reactor 22, is disposed in the power circuit between the output of power converter 14 and supply mains 12. However, the use of an inductor increases, the cost and size of the power transfer system, and increases electrical losses. Alternatively, one of power converter 14 and supply mains 12 can be disconnected prior to connecting the other, i.e. a xe2x80x9cbreak before makexe2x80x9d transfer or capture. However, such a procedure causes an undesirable bump in the power delivered to the load due to the inherently slow response time of contactors 16 and 18. In particular, a contactor ordinarily takes about 50 milliseconds to close. Recently, medium power solid state switching devices, such as IGCTs have been developed and used in the output section of variable speed motor drives. However, a reliable method of taking advantage of the high speed and commutation ability of such devices in transfer and capture applications is not known.
A first aspect of the invention is an apparatus for switching an AC motor between two power supplies having instantaneous voltage differences, comprising a first switch mechanism adapted to be coupled to a first power supply and a motor to selectively couple the motor to the first power supply, a second switch mechanism adapted to be coupled to a second power supply and the motor to selectively couple the motor to the second power supply, a current detector adapted to be coupled to the first power supply to monitor output current of the first power supply, and a controller coupled to the first switch mechanism and the second switch mechanism and configured to control the status of the first switch mechanism and the second switch mechanism. With the first switch mechanism in a closed state and the second switch mechanism in an open state to allow the motor to run under power from the first power supply, the controller is operative to close the second switch mechanism in response to a transfer signal and subsequently place the first power supply in an idle mode when the current detector indicates that an instantaneous current in the output of the first power supply has changed by a predetermined threshold amount to transfer the motor to run under power from the second power supply.
A second aspect of the invention is an apparatus for switching to an AC motor between two power supplies having instantaneous voltage differences, comprising a first switch mechanism adapted to be coupled to a first power supply and a motor to selectively couple the motor to the first power supply, a second switch mechanism adapted to be coupled to a second power supply and the motor to selectively couple the motor to the second power supply, a voltage detector adapted to be coupled to the first power supply to monitor the output voltage of the first power supply, and a controller coupled to the first switch mechanism and the second switch mechanism and configured to control the status of the first switch mechanism and the second switch mechanism. With the first switch mechanism in a closed state, the second switch mechanism in a closed state and the first power supply in an idle mode, the controller being operative to open the second switch mechanism and subsequently activate the first power supply when the voltage detector indicates that an output voltage of the motor changes by a predetermined threshold amount to capture the motor to run under power from the first power supply.
A third aspect of the invention is a method of switching an AC motor between two power supplies having instantaneous voltage differences comprising the steps of coupling a first power supply to a motor to power the motor with the first power supply, coupling a second power supply to the motor while the first power supply is coupled to the motor, and placing a first power supply in an idle mode when output current of the first power supply changes by a predetermined amount to power the motor with the second power supply.
A fourth aspect of the invention is a method of switching an AC motor between two power supplies having instantaneous voltage differences comprising the steps of coupling a second power supply to a motor to power the motor with the second power supply, coupling a first power supply to the motor while the second power supply is coupled to said motor and the first power supply is in an idle mode, uncoupling the second power supply from the motor, and activating the first power supply when output voltage of the first power supply changes by a predetermined amount to power the motor with the first power supply.
A fifth aspect of the invention is an apparatus for switching power supplies to an AC motor comprising, an AC motor, a variable frequency AC motor drive, a first contactor coupled to the variable frequency AC motor drive and the motor to selectively couple the motor to the variable frequency AC motor drive, supply mains, a second contactor coupled to the supply mains and the motor to selectively couple the motor to the supply mains, a current detector coupled to the variable frequency AC motor drive to monitor the output current of the variable frequency AC motor drive, and a controller coupled to the first contactor and the second contactor and configured to control the status of the first contactor and the second contactor. With the first contactor in a closed state and the second contactor in an open state to allow the motor to run under power from the variable frequency AC motor drive, the controller is operative to close the second contactor in response to a transfer signal and subsequently place the AC motor drive in an idle mode when the current detector indicates that an instantaneous current in an output of the variable frequency AC motor drive changes by a predetermined amount to transfer the motor to run under power from the supply mains.
A sixth aspect of the invention is an apparatus for switching power supplies to an AC motor, comprising a variable frequency AC motor drive, an AC motor, supply mains, a first contactor coupled to the variable frequency AC motor drive and the motor to selectively couple the motor to the variable frequency AC motor drive, a second contactor coupled to the supply mains and the motor to selectively couple the motor to the supply mains, a voltage detector coupled to the variable frequency AC motor drive to monitor the output voltage of the variable frequency AC motor drive, and a controller coupled to the first contactor and the second contactor and configured to control the status of the first contactor and the second contactor. With the first contactor in a closed state, the second contactor in a closed state and the variable frequency AC motor drive in an idle mode, the controller is operative to open the second contactor and subsequently activate the variable frequency AC motor drive when the voltage detector indicates that an output voltage of the variable frequency AC motor drive changes by a predetermined amount to capture the motor to run under power from the variable frequency AC motor drive.
A seventh aspect of the invention is an apparatus for transferring an AC motor between two power supplies having instantaneous voltage differences comprising, means for coupling a first power supply to a motor to power the motor with the first power supply, means for coupling a second power supply to the motor while the first power supply is coupled to said motor, and means for placing the first power supply in an idle mode when output current of the first power supply changes by a predetermined amount to power the motor with the second power supply.
An eight aspect of the invention is an apparatus for transferring an AC motor between two power supplies having instantaneous voltage differences comprising, means for coupling a second power supply to a motor to power the motor with the second power supply, means for coupling a first power supply to the motor while the second power supply is coupled to the motor and the first power supply is in an idle mode, means for uncoupling the second power supply from the motor and means for activating the first power supply when the output voltage of the first power supply changes by a predetermined amount to power the motor with the first power supply.